1. Field of the Invention
The present invention relates to a display device using light-emitting elements in a pixel portion, and more particularly to a display device using light-emitting elements typified by organic electroluminescence (EL) elements in a pixel portion, and is provided with a video data correction circuit for correcting video data correspondingly to the degradation of the light-emitting elements. In addition, the invention relates to a display panel where a light-emitting element such as an EL element is disposed in each pixel and a video data correction circuit for correcting the degradation of the light-emitting element is provided. Further, the invention relates to an electronic appliance provided with such a display device.
2. Description of the Related Art
In recent years, a display device using light-emitting elements where a semiconductor thin film is formed over an insulator such as a glass substrate, in particular an active matrix light-emitting device using TFTs (Thin Film Transistors) has been in widespread use. In the active matrix light-emitting device using TFTs, several hundred thousand to several million TFTs are disposed in a pixel portion where pixels are arranged in matrix, with which electric charge of each pixel is controlled for displaying images.
Further, in addition to the pixel TFTs which constitute the pixels, a driver circuit is simultaneously formed on the periphery of the pixel portion by using TFTs, which greatly contributes to the downsizing and lower power consumption of the device. Accordingly, the display device using light-emitting elements has become an essential device for a display portion of mobile devices and the like of which applications are increasing in recent years. In addition, by a crystallization technique for crystallizing a semiconductor film such as amorphous silicon over a glass substrate at low temperature, a high added value has been achieved such as a so-called SOG (System On Glass) in which a CPU and other modules are mounted over a glass substrate.
As an alternative display device for a liquid crystal display device (LCD), there is a display device having a display panel where a light-emitting element is disposed in each pixel and a peripheral circuit for inputting signals to the panel, which displays images by controlling the light emission of the light-emitting element.
Such a display device has a control circuit which converts a received video signal to the video data capable of displaying gray scales in the pixels of the display panel and outputs the video data to the panel together with a panel control signal. In the display panel of the display device, two or three TFTs (Thin Film Transistors) are typically disposed in each pixel, and by controlling on/off of these TFTs, current supplied to the light-emitting element in each pixel, namely the luminance and light emission/non-light emission of the light-emitting element in each pixel are controlled. Further, in the peripheral portion of the pixel portion of the panel, a driver circuit is provided for controlling on/off of the TFTs in each pixel. Such a driver circuit is constituted by TFTs which are formed simultaneously with the TFTs in the pixel portion. These TFTs may be either n-channel TFTs or p-channel TFTs.
At this time, in the case where an EL element or the like is used as the light-emitting element, current is constantly supplied and thus flows to the EL element in the period in which the EL element emits light. In the case where light emission of the EL element is performed with a current supply, luminous efficiency of R (Red), G (Green) and B (Blue) relatively to a driving current differs from each other depending on the material used for the organic EL element. Moreover, the luminous efficiency changes with time and degrades as the cumulative light-emission period (total light-emission period) becomes longer, and the degradation characteristics with time differ depending on each light-emitting material. Accordingly, the property of the EL element per se degrades by the long period of light emission, which results in changes in the luminance characteristics. That is, when comparing an EL element which has degraded and an EL element which has not degraded, luminance difference occurs even when current is supplied with the same voltage from the same current supply source. In a display device using EL elements and the like, white light is expressed by the total emission state of the whole three primary colors of RGB; therefore, reddish or bluish white is displayed in accordance with changes in the light-emission state due to the degradation of each color with time. As a result, such a problem is posed that white balance is disrupted.
Therefore, among display devices using light-emitting elements such as EL elements, there is a display device provided with a video data correction circuit which regularly corrects video data signals for driving a pixel of which EL element has degraded, with which the light-emission time or the light-emission time and intensity of each EGB pixel is detected by regularly sampling video data signals, and the cumulative detected values are compared with the prestored data on changes with time of the luminance characteristics of the EL elements in order to keep uniform display screen without causing luminance unevenness even when EL elements in some pixels have degraded.
Note that sampling in this specification means the operation in which the light-emission time or the light-emission time and intensity of each color (RGB in this specification) of each pixel is regularly detected using video signals and the detected values are accumulated.
As such a video data correction circuit, for example, there is a self-luminous display device having a degradation correction function which is invented by the present applicant and disclosed in Patent Document 1. FIG. 13 shows a block diagram of such a video data correction circuit. The video data correction circuit includes I: counter unit, II: memory circuit unit and III: signal correction unit. I includes a counter 1302, II includes a volatile memory 1303 and a non-volatile memory 1304 and III includes a correction circuit 1305 and a correction data storage unit 1306. In the video data correction circuit, a first video signal 1301A as a pre-correction video data signal (video data for driving a pixel of which EL element has degraded) is corrected by the signal correction unit III, which is then supplied to a display device 1307 as a second video signal 1301B as a corrected video data signal.
In this video data correction circuit, the second video signal 1301B as a corrected video data signal which is regularly (for example, per second) supplied to the display device 1307 is sampled, and light emission/non-light emission of each pixel is counted by the counter 1302. The cumulative number and time of light emissions of each pixel counted therein are sequentially stored in the memory circuit unit II (hereinafter referred to as the cumulative time data). The memory circuit is desirably constructed using a non-volatile memory as the number of light emissions is accumulated; however, the non-volatile memory generally has a limitation in the number of data writings thereto. Thus, in the device of FIG. 13, data is stored by using the volatile memory 1303 during operation of the self-luminous device while data is written into the non-volatile memory 1304 at regular intervals (for example, every hour or at every shut down time of the power source). That is, upon the next power-on time, the light-emission time or the light-emission time and intensity of EL elements is counted again.
[Patent Document 1] Japanese Patent Laid-Open No. 2002-175041
Here, in order to write the cumulative time data from the counter 1302 into the volatile memory 1303 in one cycle of a reception clock (which corresponds to the clock for receiving video data in the video data correction circuit in this specification), the memory is accessed at the following 4 timings: the write-in operation of the cumulative time data to the volatile memory; the readout operation for outputting the cumulative time data on R from the volatile memory to the signal correction unit; the readout operation for outputting the cumulative time data on G from the volatile memory to the signal correction unit; and the readout operation for outputting the cumulative time data on B from the volatile memory to the signal correction unit. At this time, there is only a short time between the write-in timing of the cumulative time data to the volatile memory and the output timing of the cumulative time data from the volatile memory to the signal correction unit. Thus, in order to prevent the write-in timing and the output timing form overlapping with each other, a time margin (blank period) is required to be provided for avoiding mixture of data.
As set forth above, in the case of accumulating/adding or multiplying the light-emission time of light-emitting elements, each operation timing is difficult to set in one cycle of a reception clock during which the write-in operation and the output operation of the cumulative time data are performed. Therefore, a time margin is desirably provided for preventing mixture of data. Additionally, in accordance with the enlargement of a panel in recent years, a volume of video data signals is increased, which requires a storage medium capable of high-speed operation. Thus, a time margin is still required.
In order to secure such a margin, it is required that a volatile memory and a non-volatile memory mounted on a circuit have higher capacity and operate at faster speed. However, the number of connection pins in the mounted circuit is increased as well as an area occupied by the circuit is increased in accordance with the increase in the number of bits, which obstructs the downsizing and realization of the lower manufacturing cost of a product. In addition, when the number of high-capacity RAMs is increased, lower power consumption becomes difficult to achieve.